a. Field of the Invention
The invention relates to a magnetorheological fluid composition and method for forming the same.
b. Description of the Related Art
Generally, a shock absorber or damper at least comprises a shock absorbing mechanism, for example, formed by a spring and an oil cylinder where the spring is used to absorb the instant vibrating energy and then releasing the energy after absorption is consumed by viscous friction of the fluid (oil) in the oil cylinder. The spring in the damper determines the capability of shock absorption but in practice the vibrating energy varies while the fluid (oil) has only a certain range of energy loading. That is, if viscous friction of the fluid is too small, it is over loaded for large energy release to have the residual energy keep vibrating. On the contrary, if viscous friction of the fluid is too large, it is not sensitive for small energy release to cause failure in shock absorption. Therefore, in order to improve the traditional damper, it is expected to have viscous friction of a fluid be varied with the magnitude of vibration. For example, a fluid having viscous friction varying with the strength of a magnetic field can be used as a smart damper.
A magnetorheological fluid generally comprises at least magnetic responsive particles and a carrier fluid where the average diameter of the magnetic responsive particles is about 0.1˜500 μm. Under no magnetic field, the magnetorheological fluid acts as a Newtonian fluid while under a magnetic field it acts as a Bingham fluid that has yield stress variation more than KPa. The viscosity of the magnetorheological fluid varies with the strength of the magnetic field applied thereon and then the state of the fluid may become solid-like which is extensively used as material for damping control, such as smart dampers or shock absorbers for various devices, especially for automobile.
However, the magnetic responsive particles in the magnetorheological fluid have larger particle size and thus Brownian motion cannot stop particle precipitation and aggregation. Therefore, various countermeasures are developed to prevent particle precipitation and aggregation. For example, a method of using surfactant(s) is used. But, such a method cannot change the density of particles and thus cannot resist particle precipitation in the carrier fluid because these magnetic responsive particles usually have much larger density than that of the carrier fluid. On the other hand, according to the prior art disclosed by U.S. Pat. No. 6,203,717, a stable magnetorheological fluid comprising organoclay is disclosed to reduce the precipitation rate of particles. However, in order to evenly blend organoclay, magnetic material such as carbonyl iron powders, and organic oil such as silicone oil, additional agents should be added. Not only production complexity but also production cost is increased. The stability of the additional agents should be considered.